Physics
Safety & Infection Control
Setting Up The Machine
H. Gee MD, FRCOG
(teaching materials provided by
David Cole, Lecturer at City
University, Birmingham)
WAVELENGTH 
LOW PRESSURE REGION
+
Sound
HIGH PRESSURE REGION
= ONE CYCLE
A sound wave
wavelength
Excess
pressure
amplitude
Amplitude & Frequency
• Frequency
– Pitch
• Amplitude
– Loudness
• the strength of the sound.
• The maximum change in
pressure due to the
soundwave
Ultrasound
Ultrasound
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High Frequency Sound
> 20 KHz (above the audible range)
Short Wavelength
Resolution  Wavelength
3.5 MHz - Wavelength = .44 mm
Frequency  Penetration 
Spatial Resolution
• The ability to display two small structures
situated close together as separate
images
objects
Image
resolved
Image
unresolved
Piezo-electric effect
• Sound  Electricity
Piezo-electric crystal
Inverse piezo-electric effect
• Electricity  Sound
Piezo-electric crystal
Ultrasound transducer
PULSE
CONVERTS ELECTRICITY INTO SOUND
PIEZO ELECTRIC CRYSTAL
ECHO
CONVERTS SOUND INTO ELECTRICITY
Pulse echo technique
multiple reflectors
Imaging Principles
Imaging Principles
How the image is produced
Summary
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Sound
Ultrasound
Piezo-electric effect
Pulse-echo technique
Imaging principles
Resolution
Infection Control
Setting up the Machine
• Two Parts
– Monitor (may be done automatically)
• Contrast
• Brightness
• Grey Scale on Screen – ALL GRADATIONS
– Imaging System
Equipment Controls
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Probe selection
Mode selection 2D, Doppler, Colour
Gain, TGC, Power
Depth and Zoom Scan angle (field size)
Focus
Measurement, freeze
(Examination selection post processing)
Probe selection
• This control is used to select from the
probes available
• For example
– Curvilinear general
– Transvaginal
– Linear array for small parts
Mode selection 2D, Doppler,
Colour
• 2D is normally selected automatically for
all examinations and the other modalities
are selected when required during the
examination.
Making the image darker or
brighter
• Power
• Gain,
• Time Gain Compensation
Safety
• Cavitation
• Heating
• Frequency
• Duration
• Intensity
• Power
Power (Output) increased
Safety implications
Set power to 35%
Use gain to control
brightness
BRIGHTER IMAGE
Power reduced
DARKER IMAGE
Gain
• Controls image brightness by
amplifying echo signals
• Completely safe
• Increased
image noise
Gain
INCREASE IN GAIN
MORE NOISE
Time gain compensation
(TGC)
Time gain compensation
(TGC)
Time gain compensation
(TGC)
TGC correct
Changing the field size
• The following controls change the
size of the field of view
– Depth
– Zoom
– (Scan angle)
Freeze
• Measurement
• Image recording
Measurement
=
•Electronic callipers
•Linear
•Circumference / area
•Ellipse / trace
•Volume measurement
•Organ specific
•Embedded charts
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Summary
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Probe selection
Mode selection 2D, Doppler, Colour
Depth and Zoom Scan angle (field size)
Gain, TGC, Power
Focus
Measurement, freeze (& cine loop)
(Examination selection post processing)
Important Controls
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Power (output, acoustic)
Gain
Time gain compensation (TGC)
Freeze
Electronic calipers